Summary of Carrying Values, Estimated Fair Values and Fair Value Measurement Levels of Financial Instruments (Detail) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financial Assets:
Cash and cash equivalents		$ 2,680,772	$ 2,528,169	$ 984,296	$ 557,850
FHLB stock	[1]	622,989	603,819
Loans, net		38,759,685	38,265,183
Financial Liabilities:
Deposits		29,235,434	29,102,163
Borrowed funds		13,343,759	12,913,679
Financial Assets:
Cash and cash equivalents		2,680,772	2,528,169
FHLB stock	[1]	622,989	603,819
Loans, net		38,462,422	38,254,538
Financial Liabilities:
Deposits		29,178,221	29,044,852
Borrowed funds		13,176,141	12,780,653
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash and cash equivalents		2,680,772	2,528,169
Financial Liabilities:
Deposits	[2]	20,172,114	20,458,517
Significant Other Observable Inputs (Level 2)
Financial Assets:
FHLB stock	[1]	622,989	603,819
Financial Liabilities:
Deposits	[3]	9,006,107	8,586,335
Borrowed funds		13,176,141	12,780,653
Significant Unobservable Inputs (Level 3)
Financial Assets:
Loans, net		$ 38,462,422	$ 38,254,538

[1] Carrying value and estimated fair value are at cost.

[2] Interest-bearing checking and money market accounts, savings accounts, and non-interest-bearing accounts.

[3] Certificates of deposit.